The present invention relates to local area networks (LANs) which enable devices with computational ability to communicate with each other and, in particular, to a wireless LAN in which the devices communicate by means of radio transmissions.
In recent years the personal computer has become an increasingly important tool in business and commerce and many workers now spend a good portion of their working day operating such computers. Similarly, business organisations are increasingly structuring their businesses to not only enable, but to oblige, their workers to access information by means of a personal computer or equivalent terminal, which is connected to a local area network which extends around or through the office environment.
Hitherto such local area networks have been provided either by electrical conductor or optical fibre and this requires the office premises to be extensively cabled. This cabling must be adjusted if, for example, partitions within an office are to be adjusted. In addition, the cabling required for a classroom or tutorial arrangement where a large number of personal computers are intended to be operated within a small areas, can be quite substantial.
Furthermore, an increasing trend in recent times has been the sale of mobile or portable devices with computational ability. These include both laptop/notebook and handheld computers. Whilst the primary impetus for the purchase of such a computer is the ability to use its computational power outside of the normal office environment, once a portable computer has been purchased, the desire arises to use the portability within the office premises so as to allow the user of the portable computer to take the computer with him and use it in the closely adjacent offices of colleagues, for example, and yet still be able to access the LAN of the business organisation, which may be spread over several adjacent buildings in "campus" style.
While this is possible by means of plug-in connectors which enable the portable computer of one operator to be plugged into the office LAN at any particular location, it is generally inconvenient since the LAN may not provide for two or more points of connection within a single office, the portable computer loses its portability, and so on.
Accordingly, the need arises for a LAN to which such portable devices can be connected by means of a wireless or radio link.
Such wireless LANs are known, however, hitherto they have been substantially restricted to low data transmission rates. In order to achieve widespread commercial acceptability, it is necessary to have a relatively high transmission rate and therefore transmit on a relatively high frequency, of the order of 1 GHz or higher. As will be explained hereafter, radio transmission at such high frequencies encounters a collection of unique problems.
One wireless LAN which is commercially available is that sold by Motorola under the trade name ALTAIR. This system operates at approximately 18 GHz, however, the maximum data transmission rate is limited to approximately 3-6 Mbit/s. A useful review of this system and the problems of wireless reception at these frequencies and in "office" environments is contained in "Radio Propagation and Anti-multipath Techniques in the WIN Environment", James E. Mitzlaff IEEE Network Magazine November 1991 pp. 21-26.
This engineering designer concludes that the inadequate performance, and the large size, expense and power consumption of the hardware needed to adaptively equalize even a 10 Mbit/s data signal are such that the problems of multipath propagation cannot thereby be overcome in Wireless In-Building Network (WIN) systems. Similarly, spread spectrum techniques which might also be used to combat multipath problems consume too much bandwidth (300 MHz for 10 Mbits/s) to be effective. A data rate of 100 Mbit/s utilizing this technology would therefore consume 3 GHz of bandwidth.
Instead, the solution adopted by Motorola and Mitzlaff is a directional antenna system with 6 beams for each antenna resulting in 36 possible transmission paths to be periodically checked by the system processor in order to locate the "best quality" path and "switch" the antennae accordingly. This procedure adds substantial bulk and cost to the system. This procedure is essentially the conversion of a multipath transmission problem into a single path transmission environment by the use of directional antennae.